It is known that numerous aircraft are equipped with a collision alert device making it possible to emit at least one alert signal in the case of a risk of the aircraft colliding with the terrain. Concerning the alert device, it may in particular be a device of TAWS (“Terrain Awareness and Warning System”) type, of EGPWS (“Enhanced Ground Proximity Warning System”) type or of GCAS (“Ground Collision Avoidance System”) type.
Such a TAWS collision alert device is generally capable of emitting various types of alerts, the triggering of which is based on different algorithms and sources of information.
A terrain avoidance system for a civilian airplane equipped with a collision alert device of TAWS type is known through the document FR-2883403. This system automatically triggers a terrain avoidance maneuver when the collision alert device emits an alert to signal that the airplane is at risk of colliding with the terrain if it maintains its current flight characteristics. Thus, as soon as an alert is emitted, the system acts automatically (without pilot intervention) on the trajectory of the airplane throughout the duration of emission of the alert, so as to improve its situation with respect to the terrain. When the collision alert device ceases to emit said alert, the avoidance system automatically interrupts the collision avoidance maneuver.
However, it is possible that, at the end of a terrain avoidance maneuver, the airplane may still not be in a satisfactory situation from a safety point of view. Indeed, in certain situations (mountainous zones in particular), depending on the position and the trajectory of the airplane at the end of the avoidance maneuver, a new risk of collision may be detected by the collision alert device a very short time after the end of the avoidance maneuver, and this may lead to the engagement of a new terrain avoidance maneuver.
Now, this new avoidance maneuver is not desirable, on the one hand, in order to effect a resumption of control of the airplane by the pilots under good conditions and, on the other hand, to avoid discomfort to the passengers, if any.